Mounting Failed?

I am a Windows user who recently switched to Mac, I bought my first Mac back in January, it was an iMac G3 with Mac OS X Tiger. I am trying to download software and applications on my Mac, but every time I do I get a message that says Mounting Failed. I looked it up and got some answers but non of them helped me. Please help me! I'm most likely going to get a more later Mac soon, but I would like to still have this around.